The invention is concerned with a method of controlling the zinc phosphate based conversion coating treatment solutions (referred to below as treatment solutions) which are used as a coating pretreatment or as an anti-rust or plastic deformation lubrication pretreatment for metals, especially steel and zinc plate.
When a zinc phosphate coating (referred to below as a coating) is formed on the surface of a metal the weight of the coating, the corrosion resistance after painting and the adhesion of the paint etc. are generally known to be affected to a considerable degree by the composition of the treatment solution employed and by the changes which occur in the composition of this solution. Hence it is very important that the composition of the treatment solution selected should have component concentrations, and especially the zinc ion and the zinc ion+nickel ion concentrations, within the preferred range and that these concentrations should be maintained within the preferred range if coatings which have the best possible properties are to be formed consistently on the surface of the metal which is being treated.
The control of the component concentrations in the treatment solution is achieved conventionally by measuring the free acid, total acid and promotor concentrations. These measurements are made using neutralization titration or redox titration methods and they can be carried out either manually or automatically. Although it is possible to control the free acid, total acid and promotor concentrations in the treatment solution using methods of this type it is very difficult to control the zinc ion and the zinc ion+nickel ion concentrations in the treatment solution to within the preferred ranges in this way.